Computer networks are often composed of many different types of connections from network interconnection.If several computer network is physically connected together and can not communicate between them, then this "interconnection" and of little practical significance.So often talk about "interconnected", it has been suggested that these inter-connected computers can communicate, that is, from a functional and logical point of view, these computer networks have formed a large computer network, orknown as the Internet, also referred to as the Internet, the Internet.
Interconnected to the network using some intermediate device (or intermediate systems), ISO terminology called relay (relay) system.According to the level where the relay system, relay system for the following five:
1. Physical layer (that is often said that the first layer, the layer L1) relay system, the repeater (repeater).
2. Data link layer (the second layer, layer L2), the bridge or bridge (bridge).
3. Network layer (Layer, Layer L3) relay system, the router (router).
4. A mixture of bridges and bridge router device (brouter) both bridge and router functions.
5. In the network layer above the relay system, the gateway (gateway).
When the relay system is the transponder, generally do not call it networking, since it is expanded to a network, and this is still a network.Of more complex high-level gateway, currently using less.So is the general discussion of network interconnection switches and routers that interconnect with the network.This article focuses on switches and routers and their differences.
2 switches and routers
"Exchange" is the highest frequency the network today, a word from the bridge to the route to the ATM until the phone system, regardless of the occasion can be applied, not quite sure what is the real exchange in the end.In fact, the word first appeared in the telephone exchange system, especially to achieve two different voice signals between the telephone exchange equipment to complete the work is telephone exchange.Therefore, from the original intention of speaking, the exchange is only a technical concept, the signal from the device to complete the forwarding entry to the exit.Therefore, and in accordance with the definition as long as all equipment can be called exchange equipment.This shows that the "exchange" is a broad term meaning, when it is used to describe the second layer of data network equipment, actually refers to a bridge device; and when it is used to describe the data network, the third layerthe equipment, but also refers to a routing device.
We often said that the Ethernet switch is actually a multi-port bridge technology based on the second level network device that the data frames from one port to another free port forwarding provides low delay, low overhead path.
Thus, within the core of the switch should have a switch fabric for communication between any two ports provide access to, or a rapid exchange of the bus, so that any port received by the data frame sent from other ports.In the actual device, the switching matrix functions often by a dedicated chip (ASIC) to complete.In addition, the Ethernet switch in the design of thinking has an important assumption, that exchange of the core very quickly, resulting in a large flow of data is not usually to produce congestion, in other words, the ability to exchange information with respect to the transfervolume of the infinite (to the contrary, ATM switch design idea is that the exchange capacity is relatively limited in terms of the amount of information transmission).
Although the Ethernet switch is based on the second layer developed from the multi-port bridge, but after the exchange has a more feature-rich, so that not only the best way to get more bandwidth, but also make the network more manageable.
The router is a network layer OSI protocol model of packet switching device (or network layer relay device), the router's basic function is the data (IP packets) sent to the correct network, including:
1.IP forwarding packets, including data reported routing and delivery;
2. Subnet isolation, broadcast storm suppression;
3. To maintain routing tables with other routers to exchange routing information, which is the basis for IP packet forwarding.
4.IP datagram simple error handling and congestion control;
5. To achieve the IP packet filtering and accounting.
The scale of the network differently, the focus of the role of the router is different.
In the backbone, the router's primary role is routing.Backbone router, must know the path to reach all the underlying network.This need to maintain large routing tables, and the connection to make changes in the status response as quickly as possible.Router failure will lead to serious problems of information transmission.
In regional networks, the router's main role is network connectivity and routing, all grass-roots networks that connect the lower unit - campus network, is also responsible for forwarding data between the underlying network.
Within the campus network, the router's main role is to separate the subnet.The early Internet is the primary unit of local area network (LAN), in which all hosts in the same logical network.With the continuous expansion of network size, evolved into a high-speed LAN backbone and router connected to multiple subnets composed of campus.In which, at a separate logical subnet, and router is the only device able to separate them, which is responsible for forwarding packets between subnets and broadcasting isolation, on the border routers are responsible for the upper network.